Gibberellin Application Improved Bunch Rot Control of Vignoles Grape, but Response to Mechanical Defoliation Varied Between Training Systems.

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Late-season bunch rot causes significant crop loss for grape growers in wet and humid climates. For 3 years (2016 to 2018), we integrated prebloom mechanized defoliation (MD) in the fruit zone and bloom gibberellin (GA) applications, either alone or in combination, into the bunch rot control program of Vignoles, a commercially valuable grape variety that is highly susceptible to bunch rot. We hypothesized that both treatments would decrease bunch rot through modification of cluster architecture or fruit zone microclimate compared with vines treated with the standard chemical control program. Grapevines were trained to two popular training systems, four-arm Kniffin (4AK) and high-wire bilateral cordon (HWC). Treatment responses varied between training systems. MD, alone or in combination with GA, reduced bunch rot incidence and severity every year on 4AK-trained vines, an effect attributed mainly to fruit zone improvements. Conversely, MD alone did not reduce bunch rot incidence on HWC-trained vines, despite significant improvements in cluster architecture (reduced number of berries per cluster and cluster compactness). GA applications were more effective than MD at reducing cluster compactness, regardless of training system. As a result, GA reduced bunch rot incidence and severity when applied alone or with MD on 4AK- and HWC-trained vines. All treatments positively improved fruit-soluble sugar concentration on both training systems, while positive effects on titratable acidity were more consistent across training systems with MD.